Group III nitride materials include gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN) and their alloys such as aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN), and aluminum indium gallium nitride (AlInGaN). These materials are semiconductor compounds that have a wide direct bandgap, which permits highly energetic electronic transitions to occur. Such electronic transitions can result in group III nitride materials having a number of attractive properties including the ability to efficiently emit blue and ultraviolet light, the ability to transmit signals at high frequency, and others. Accordingly, group III nitride materials are being widely investigated in many semiconductor device applications, including microelectronic devices such as transistors, and optoelectronic devices such as laser diodes and light emitting diodes (LEDs).
Group III nitride materials have been formed on a number of different substrates including sapphire, silicon (Si), and silicon carbide (SiC). Semiconductor structures, such as doped regions, may then be formed within the group III nitride material region. There are many advantages of growing group III nitrides, such as GaN, on Si substrates, an important one of which is the integration with Si-based electronics and the availability of very large area substrates. Previously, however, semiconductor structures having group III nitrides formed on Si substrates have presented significant drawbacks. Such structures have been complicated and expensive to fabricate. Moreover, light emitting optoelectronic devices having group III nitrides formed on silicon substrates are less efficient than such devices formed on sapphire substrates. In optoelectronic applications, Si is approximately 45% absorbing in the ultraviolet (UV) region, while sapphire is totally transparent (see, Aspnes, et al. Phys. Rev. B 27, 985 (1983)). Thus, a light-emitting optoelectronic device based on group III nitrides will be less efficient if Si(111) is used as a substrate than if sapphire is used as a substrate.
The growth of group III nitrides, including GaN, is most commonly accomplished by heteroepitaxy using methods of metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). The substrates employed are generally sapphire and α-SiC(0001), which have lattice mismatches of 16% and 3.6% respectively with GaN. Coupled with mismatches in thermal expansion coefficients, the misfit dislocations produced in GaN during heteroepitaxial growth pose a limitation to the ultimate performance of nitride-based electronics. Various growth schemes involving patterned substrates have been developed to improve the dislocation density. These growth schemes include, for example, epitaxy by lateral overgrowth (ELOG), which is described in Kato, et al. J. Cryst. Growth 144, 133 (1994), and pendeoepitaxy (PE), which is described in Linthicum et al, Appl. Phys. Lett. 75, 196 (1999). Nevertheless, the quest for lattice-matched substrates continues. Bulk GaN crystals grown under high pressures, as described by Porowski, J. Cryst. Growth 189/190, 153 (1998), have been used as substrates. Such substrates, however, are hampered by their small size. Another approach to homoepitaxy is the growth of thick GaN layers by hydride vapor phase epitaxy (HVPE), which is described by Molnar, et al., J. Cryst. Growth 178, 147 (1997). These substrates, however, suffer from poor crystallinity and the highly strained layers often develop cracks and other undesirable morphologies.
Kinoshita et al. Jpn. J. Appl. Phys., pt. 2, 40, L1280 (2001) have reported the growth of single crystals of zirconium diboride, ZrB2(0001) to provide an electrically conductive lattice-matched substrate for GaN growth. ZrB2 has a hexagonal structure with lattice constants a=3.169 Å and c=3.530 Å. The in-plane lattice constant has about 0.6% mismatch with that of GaN (a=3.189 Å). The thermal expansion coefficients along [1010] on the basal plane are also well-matched between ZrB2 and GaN, being 5.9×10−6 K−1 and 5.6×10−6 K−1 respectively. While these similarities in thermal properties between ZrB2 and GaN suggest that the use of ZrB2 (0001) as a substrate for the growth of GaN films may lead to a reduction of both dislocation density and biaxial strain in the GaN, significant drawbacks still limit the use of ZrB2 as a substrate for the growth of GaN films. One such drawback is the high temperature required to prepare single crystals of ZrB2. Preparation of these crystals requires very high temperatures since the melting point of ZrB2 is 3220° C. A float-zone method has been developed, as described by Otani, et al., J. Cryst. Growth 165, 319 (1996), in which a 1-cm diameter rod was isostatically pressed at 1700° C. from ZrB2 powder and melted in a floating zone by radio frequency (RF) heating. The molten zone was about 0.5 cm long and a growth rate of 2-3 cm per hour was obtained, as described by Otani et al. and Kinoshita et al. The ZrB2 single crystals thus grown, however, have size limitations.
A typical size of such a crystal of ZrB2 is 1 cm in diameter and 6 cm long. Successful epitaxial and strain-free GaN and AlN growth on such single crystals of ZrB2 using MBE and MOCVD, respectively, have been reported, respectively by Suda et al., J. Cryst. Growth 237-239, 1114 (2002) and Liu et al., Appl. Phys. Lett. 81, 3182 (2002). However, the size limitation of the ZrB2 substrate remains an unresolved issue.